Association of Wet-Bulb Globe Temperature
with heat-related illness hospitalizations
in Japan: a time-stratified, case-crossover
study

Association of Wet-Bulb Globe Temperature
with heat-related illness hospitalizations
in Japan: a time-stratified, case-crossover
study

Keywords Wet-Bulb Globe Temperature, Heat stroke, Heat related illness, Global warming

Abstract

Background Heat-related illnesses are a serious public health concern and are exacerbated by global warming. Wet-
Bulb Globe Temperature (WBGT) is widely used as a heat stress indicator, but its clinical impact remains unclear. This

study aimed to investigate the association between hourly variations in WBGT and the incidence of hospitalizations
for heat-related illness in Japan using a nationwide database. By incorporating individual-level clinical data and
performing stratified analyses, we sought to provide a more granular understanding of how heat exposure affects the
risk of heat-related illness requiring hospitalization.
Methods We conducted a time-stratified, case-crossover study using data collected from July to September in 2020
and 2021 in the Heatstroke STUDY registry. The inclusion criteria were patients registered in the Heatstroke STUDY
registry, specifically hospitalized patients with heat-related illness who were transported to participating hospitals
during the study period. Hourly WBGT values were assigned based on the nearest monitoring station to each hospital.
Conditional logistic regression and distributed lag models were used to estimate associations between WBGT and the
risk of hospitalization.
Results A total of 1,653 heat-related illness hospitalizations were analyzed. The mean patient age was 67.9 years;
67.6% were male. Each 1 °C increase in WBGT at onset (hospital arrival) was associated with a significantly increased
risk of hospitalization (OR 1.10, 95% CI: 1.05–1.15). The cumulative effect over the prior six hours was also significant
(OR 1.56, 95% CI: 1.50–1.62). Compared with WBGT<25 °C, adjusted ORs were 3.39 (25–27 °C), 8.81 (28–30 °C), and
22.10 (≥31 °C). Stratified analyses suggested stronger associations among several subgroups; however, only patients
with mental disorders showed statistically significant effect modification, whereas elevated WBGT posed a risk across
all groups.

Yuka Yamamura1

, Takashi Hongo2,5*, Tetsuya Yumoto2

, Fumiya Sasai1

, Kohei Tokioka2

, Takafumi Obara2
,

Tsuyoshi Nojima2

, Jun Kanda3

, Shoji Yokobori4

, Hiromichi Naito2

, Takashi Yorifuji1

and Atsunori Nakao2

https://doi.org/10.1186/s12245-025-01112-x

2026

peri irawan

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